A. Field of the Invention
This invention relates to controlling an AC source to directly drive a low voltage device.
B. Prior Art
It has been desired to provide circuitry for controlling a high voltage AC source in order to directly drive a low voltage device. For such applications, the circuitry should dissipate a negligible amount of power and not be required to filter the ripple. The prior art has left much to be desired in providing a solution to this application particularly where there is further requirement that the circuitry be inexpensive and fit in a small space. The low voltage device to be driven may be a low voltage incandescent light bulb which may be turned on for a substantial amount of time to indicate the timing out of a coulometric timing cell as described, for example, in U.S. Pat. Nos. 3,355,731; 3,711,751; and 3,769,557. Once the light bulb has been turned on, it would be damaged by any one short time duration increase in voltage beyond its rating.
While transformers have been used to directly drive a low voltage device from a high voltage AC source, transformers are relatively costly and are objectionable here for weight and space considerations. While dropping resistors may take up less space than a transformer, they have substantially high power dissipation when used to drive a low voltage device requiring power such as an incandescent light bulb. Conventional SCR circuits also have many shortcomings for this type of drive application. Specifically, once turned on, a light bulb tracks the applied voltage and thus it is critical that the maximum peak potential applied to the light bulb not exceed its maximum voltage rating. However, an SCR circuit is sensitive to electrical disturbances on the line which may come, for example, from extraneous signals such as welding torches, circuit breakers, etc. These extraneous signals may turn on the SCR during a high voltage portion of the AC signal. Accordingly, a substantially high voltage may be applied to the low voltage device for no more than 1/2 cycle which would damage the low voltage device since it tracks its applied voltage.
As a result of cost and space considerations, voltage regulated power supplies would not be suitable for this drive application particularly since voltage regulated supplies have substantial and costly filtering while ripple is tolerated in driving a low voltage device such as a light bulb.
It is therefore a general object of this invention to provide a semiconductor circuit timed by a coulometric cell which controls a high voltage AC source to drive a low voltage device without any appreciable power dissipation within the circuit and without requiring filtering.